The present invention relates generally to semiconductor processing, and particularly to mixing ultrapure liquid reagents with great accuracy.
On-site Ultrapurification
The present inventors have developed a method for preparing ultra-high-purity liquid reagents (including aqueous HF, HCl, NH.sub.4 OH, NH.sub.4 F, and HNO.sub.3) in an on-site system located at the semiconductor wafer production site. In the example of aqueous ammonia, this is performed by: drawing ammonia vapor from a liquid ammonia reservoir, and scrubbing the filtered vapor with high-pH purified water (preferably ultrapure deionized water which has been allowed to equilibrate with the ammonia stream). This discovery permitted conversion of commercial grade ammonia to ammonia of sufficiently high purity for high-precision manufacturing without the need for conventional column distillation. The drawing of the ammonia vapor from the supply reservoir serves by itself as a single-stage distillation, eliminating nonvolatile and high-boiling impurities, such as alkali and alkaline earth metal oxides, carbonates and hydrides, transition metal halides and hydrides, and high-boiling hydrocarbons and halocarbons. The reactive volatile impurities that could be found in commercial grade ammonia, such as certain transition metal halides, Group III metal hydrides and halides, certain Group IV hydrides and halides, and halogens, previously thought to require distillation for removal, were discovered to be capable of removal by scrubbing to a degree of ultrapurification which is adequate for high-precision operations. This is a very surprising discovery, since scrubber technology is traditionally used for the removal of macro-scale, rather than micro-scale, impurities. Details of such systems are described in copending U.S. application Ser. No. 08/179,001 filed Jan. 7, 1994, now U.S. Pat. No. 5,496,778, which is hereby incorporated by reference, and in copending provisional applications 60/023,358, 60/038,711 and 60/038,712 all filed Jul. 7, 1995. and all hereby incorporated by reference.
The present inventors have also developed systems for on-site purification of hydrogen peroxide, and details of such systems are described in copending U.S. provisional application 60/017,123 filed Jul. 7, 1995 which is hereby incorporated by reference.